Personally I don't think its good form to be using the customer's water really. You should be self sufficient IMO. However, I suppose it depends on the customer. Its not the cost as such, because this is negligible but it is the impression it gives.
I know a window cleaner who doesnt have a system (he uses ladders), but he DOES have a pole and a filter that clips straight onto the tap. He only uses the pole for difficult bits like conservatory roofs etc. and the customers dont mind.
I've tried telling him that he should just go the whole hog and turn 100% waterfed pole (for safety if nothing else), but he's not having it.
Each to his own.